The invention relates to a flat solar energy collector which converts solar radiation into heat.
Flat-plate solar energy collectors are well known in the art for the conversion of solar radiation into heat. These collectors consist of an absorber, heat insulation on the back side of the absorber, a covering above the absorber which is transparent for the incoming solar radiation, and a box-type or frame construction which connects the parts. The absorber is generally a metallic surface which is in close contact with a pipe or tube system through which a heat transfer medium flows, transporting the radiant energy that has been converted into heat to the user. The absorber is usually blackened on the side which faces the sun so as to provide maximum efficiency in absorbing solar energy for heating a heat absorbing or transfer medium passing through the pipe or tube system. It is here that the radiation is converted into heat.
The absorber, heated by solar radiation, not only transmits the heat to the heat transfer medium but also loses heat to surrounding areas and steps have been taken in the past to reduce these losses, which occur particularly by convection and conduction. For example, the side of the collector opposite to the incident solar radiation can be protected against heat losses in a simple way. Conventional insulating materials, e.g., glass wool asbestos or styrofoam, in appropriate strength, can be placed on the back side of the absorber thereby providing good heat insulation at low costs.
It is more difficult to protect the side of the absorber exposed to solar radiation against heat losses. Thermal insulating devices installed on the side of the absorber are required, as far as possible, to permit the radiation to pass through these thermal insulating devices without hindrance, i.e. they must be substantially transparent for solar radiation. Therefore glass panes, plastic panes or plastic sheeting is used as a covering.
In addition to heat losses occurring at the back of the absorber and through the transparent covering, heat losses also occur over the edge. A solar collector with a high efficiency is therefore characterized by the fact that a large part of the incident solar radiation reaches the absorber through the transparent covering, and is absorbed, as far as possible, and converted into heat which is mostly usable heat whereas the heat losses via front and back as well as via the edge are small.
In addition to the aforementioned radiation, physical and heat transfer requirements, a good solar energy collector should also have other qualities. Significantly, its working life should be long. The materials being used must be selected so that they can withstand thermal stresses and be resistant to corrosion. A particularly critical point is the nature of the space between the absorber and transparent covering. If the collector heats up when exposed to solar radiation, then the temperature will rise within this space thereby causing the air within this space to expand with the resultant flow of the air towards the outside until a pressure equalization is achieved with the atmosphere, unless this space is completely gas-tight with respect to the outside. After the air within said space cools, air then flows from outside to inside. One could also say that "the collector breathes". Thus, dangerous water condensation can develop within the space between the absorber and the transparent covering. Condensed water not only reduces the efficiency of the collector but it is also corrosive to the blackening of the absorber, the absorber sheeting, the sealing compounds, etc.
It is possible to avoid the formation of condensed water by airing the space between the absorber and the transparent covering. However, in the long run, this will create dust and dirt on the parts adjoining this space, i.e. the transparent covering and the absorber thereby reducing the efficiency of the device. Another method is to seal the space against gas exchange between the absorber and the transparent covering with respect to the outside. This task proves to be difficult since all organic plastics allow water vapor to diffuse to a more or less high degree. Adhesion surface areas should be thin and wide.
In principle, there are, so far, two possible methods for obtaining a gas-tight, and significantly, a water vapor-tight seal for the space between the absorber and the transparent covering, which include:
1. The metallic absorber is glued over bars, or spacing member 5 (preferably metallic) to the transparent covering 1 (usually glass panes), as illustrated in FIG. 1; and
2. The absorber is placed within a case defined by walls 2 and 8 which is connected with the transparent covering 1 is a gas-tight manner. The case is generally made of a metal (see FIG. 2).
The aforementioned types of construction have disadvantages. In the first method (see FIG. 1), the thermal contact between the hot absorber and the edge structure is very high, therefore resulting in significant heat losses from the edge of the absorber plate. In the second method (see FIG. 2), the absorber is well-insulated thermally, from the edge structure, but it is difficult to conduct the inflow and outflow of the heat transfer medium through the wall or the bottom of the base. This should be done in such a manner wherein only an insignificant heat transfer results from the inflow and outflow ducts to the metallic case, while at the same time maintaining a gas-tight seal. Such constructions are, therefore, expensive to produce.
Another disadvantage is the great volume of gas present particularly when open, porous insulating materials, e.g., glass wool, are used.
When the air space in the solar collector heats up under the influence of solar radiation, the developing air pressure can only be minimally reduced by pushing out the bottom of the case and the glass covering towards the outside. This increase in volume is too low with respect to the great initial volume of the air space. The arrangement according to FIG. 1, is definitely more advantageous. Arching of the covering and the absorber decreases the pressure because there is a great change in volume--relative to the initial volume.